1. Field of the Invention
The present invention relates to an array type antenna for radioelectrical or ultrasound waves, protected against jamming in reception. This antenna has radiating elements or groups of radiating elements individually fitted out with active or passive amplitude-controlled and phase-controlled modules enabling an aiming operation, at both transmission and reception, by analog beam-forming, and adaptive computational beam-forming means carrying out reception anti-jamming.
2. Description of the Prior Art
Hereinafter, amplitude-controlled and phase-controlled active or passive modules shall be simply called controlled modules, purely with a view to simplification.
An array antenna consists of an assembly of radiating elements distributed in an array, most usually a surface array, with a mesh size of about .lambda./2 , i.e. half of the wavelength of the radiation transmitted or received, to prevent the appearance of array lobes that disturb the directivity of the antenna.
The sizing of an antenna is a function of the amplitude of the signal to be received, namely of the signal-to-noise ratio desired at reception and the desired angular resolution.
In most cases, the signals to be received are characterized by a uniform surface density of power at the place of reception so that the power of the useful signal received increases as the useful surface area of the antenna.
The angular resolution for its part is defined in each direction by the linear dimension L of the antenna in the direction considered related to the wavelength .lambda. in the relationship .lambda./L, the solid angle resolution being defined in the ratio .lambda..sup.2 /S, where S is the surface area of the antenna.
In practice, the requirement of fineness of angular resolution is more difficult to meet than that of a high signal-to-noise ratio. Hence, if no compromise is accepted, the end result is an excessive number of radiating elements. Since, for reasons of cost, it is sought to limit the number of radiating elements of an array antenna to the utmost extent, it is worthwhile to curb this excess number by leaving gaps in the meshwork of radiating elements on the surface of an array antenna. The array antenna is then said to be a thinned array or a sparse array depending on whether the number of missing radiating elements is smaller than or greater than the number of radiating elements present.
In a thinned array or sparse array antenna, the absence of certain radiating elements means that the mesh size of about .lambda./2 no longer prevails. This leads to the appearance of array lobes if the arrangement of the missing radiating elements is periodic or to the appearance of scattered lobes if this arrangement is random. It is important to reduce these array lobes and scattered lobes to the utmost possible extent.
An array antenna may have mechanical aiming or electronic aiming. When the aiming is electronic, it is done at transmission by an analog beam-forming operation whereas at reception it may be done either by an analog beam-forming operation or by a computational beam-forming operation.
Analog beam-forming requires that the radiating elements of the antenna or groups of radiating elements should be fitted out individually with amplitude-controlled and phase-controlled active or passive elements used to orient the plane of the waves transmitted or received in the desired direction. Analog beam-forming has the advantage of working both in transmission and in reception. If necessary, an amplitude control or a distribution array enables an amplitude weighting operation.
Computational beam-forming consists in digitizing the signals received by each of the radiating elements after they have been demodulated coherently and then in phase-shifting them individually and taking a weighted sum thereof by computer to orient the plane of the received waves in the desired direction. It has the advantage of providing great flexibility to the beam-forming operation since, by computation, it is possible to simultaneously form several beams aiming in different directions. When it is adaptive, it furthermore enables the performance of anti-jamming operations by adjustment of the positions of the zeros in the radiation pattern. However, it has the disadvantage of not being usable at transmission, requiring costly equipment for the digitization of the signals of the radiating elements and requiring a very large number of computation operations.
To limit the cost of a computational beam-forming operation, the idea has arisen of dividing the array of the antenna into sub-arrays and carrying out the computational beam-forming operation in a reduced form not on individual signals of the radiating elements but on signals delivered individually by sub-array groupings of the radiating elements. The mesh size of the antenna at about .lambda./2 is no longer maintained. This leads to the appearance of array and/or scattered lobes so that the reduced beam-forming leads to poor performance characteristics of the antenna on a wide angular field. However, it remains interesting, in its adaptive form, for specific angular anti-jamming operations for, in order that such operations may be efficient, it is not necessary for the beam-forming operation to cover a large number of reception signals.
Given these considerations and owing to the fact that an array antenna is often used both at transmission and at reception, it is usual to fit out the radiating elements of an array antenna, individually or by groups, with controlled modules enabling an operation of aiming by analog beam-forming, and to assemble, in reception, the radiating elements of the antenna into sub-arrays to carry out an anti-jamming operation by a reduced adaptive computational beam-forming operation, the radiating elements being assembled in reception into surface sub-arrays and the reduced adaptive computational beam-forming operation being done in both directions of aim, relative bearing and elevation.
The reduced adaptive computational beam-forming operation generates a radiation pattern whose major lobe preserves the aiming direction produced by the controlled modules but whose zeros are shifted towards the jammers. This is done chiefly by playing on the relative amplitudes and, possibly, on a secondary basis, by bringing into play the relative phase shifts dictated on the reception signals of the sub-arrays. With the total energy being preserved, this radiation diagram retains the drawback of having array lobes with discrete angular positions or scattered lobes depending on whether the organization of the surface sub-arrays in the antenna is periodic or random, for the sub-arrays necessarily have phase centers that are spaced out by a distance greater than or equal to .lambda. expressing a sub-sampling of the surface of the antenna.
To limit this drawback, it has been proposed in the U.S. Pat. No. 5,675,343 filed by the present Applicant to perform an aiming operation, on an array antenna, by analog beam-forming. This operation is complemented by an operation of anti-jamming in reception by means of two reduced computational beam-forming operations applied to the reception signals of the radiating elements fitted out with controlled modules, assembled in two sets of parallel linear sub-arrays oriented in two different directions, and by means of a non-linear combination of the two reception signals resulting from the two reduced computational beam-forming operations. This procedure leads to a reduction in the disturbances that come in through the minor lobes, and especially through the array lobes in the case of a thinned antenna. However, the non-linear processing of the grouping of the reception signals resulting from the two reduced computational beam-forming operations may generate intermodulation products if several signals are present in the same range gate of one and the same angular cell: for example a target signal and a clutter signal. This drawback may be circumvented by a Doppler filtering upline with respect to the grouping of the two reception signals resulting from the reduced computational beam-forming operations, but this means that it is necessary to double the Doppler filtering equipment.
An aim of the present invention is an array antenna with aiming by analog beam-forming and anti-jamming at reception by means of reduced adaptive computational beam-forming operations leading to a low level of the minor lobes or scattered lobes, whether this array antenna is a full, thinned or sparse array antenna.